Exposure to ultraviolet light, in particular, ultraviolet light B (UVB, ultraviolet light in the wavelengths ranging from 290-320 nm) is known to be a major causative factor in the development of skin cancer. Epidermal cells express a variety of protective molecules that function by absorbing UVB light. Sun exposed tissue is also protected by detoxification and repair enzymes, as well as other antioxidant molecules. The precise mechanisms by which ultraviolet light induces tissue damage are not clear. It has been suggested that cytotoxic reactive oxygen intermediates generated by UVB light in the skin are responsible for causing DNA damage leading to cancer. We have discovered that UVB light rapidly stimulates the production of hydroperoxides by mouse and human keratinocytes in a process that does not require intact cells. Greater amounts of hydroperoxides are produced in calcium-differentiated keratinocytes when compared to growing keratinocytes. Purification studies using homogenates of keratinocytes identified a major protein responsible for generating hydrogen peroxide in the cells in response to UVB light. This UVB light/peroxide generating activity of requires oxygen and is eliminated by heat denaturation. Unexpectedly, sequence analysis identified this protein as catalase, an enzyme known to be responsible for the degradation of intracellular hydrogen peroxide. It is well recognized that catalase also possesses peroxidatic activity and UVB light stimulates this function of the enzyme. In this regard, we found that inhibition of the hydrogen peroxide metabolizing activity of catalase with 3-amino-1,2, 4-triazole or azide markedly enhanced the ability of UVB light to generate hydroperoxides. We hypothesize that catalase is an important mediator of UVB light-induced oxidative stress and DNA damage in keratinocytes. Our specific aims are to determine if catalase mediates ultraviolet light-induced oxidative stress and DNA damage in growing and differentiated keratinocytes and to evaluate the role of the enzyme in UVB-induced carcinogenesis using the mouse skin model. Our proposed studies will provide information on the mechanisms by which ultraviolet light induces DNA damage in the skin and should also provide information on the potential role of catalase in ultraviolet light induced skin cancer.